1. Field of the Invention
This invention relates to a semiconductor integrated circuit device. More particularly, it relates to a semiconductor integrated circuit device comprising memory cells including magneto-resistive effect elements.
2. Description of the Related Art
A variety of memories adapted to store information on the basis of novel theorems have been proposed in recent years. As such memories, magnetic random memories that utilize the magneto-resistive effect, the tunneling magneto-resistive (to be referred to as TMR hereinafter) effect proposed by Roy Scheuerlein et al. in particular, are known.
(Reference Paper)
ISSCC2000 Technical Digest p. 128 “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”
A magnetic random access memory stores information in the form of “1s” and “0s” by means of TMR elements. As shown in FIG. 18 of the accompanying drawing, the TMR element has a structure of sandwiching an insulating layer (tunnel barrier layer) between a pair of magnetic layers (ferromagnetic layers). The information stored in a TMR element is judged on the basis if the spinning directions of the paired magnetic layers are in parallel or in opposite parallel.
Referring to FIGS. 19A and 19B, “parallel” refers to a situation where the spinning directions of the paired magnetic layers are identical, whereas “opposite parallel” refers to a situation where the spinning directions of the paired magnetic layers are opposite relative to each other (arrows indicate spinning directions).
Normally, an antiferromagnetic layer is arranged at the side of one of the paired magnetic layers. The antiferromagnetic layer is a member that is adapted to facilitate the operation of writing information by fixing the spinning direction of the one magnetic layer and reversing the spinning direction of the other magnetic layer.
The tunnel resistance of the insulating layer (tunnel barrier layer) sandwiched between the paired magnetic layers is minimized when the spinning directions of the two magnetic layers are in parallel as shown in FIG. 19A. This state may be “1” state.
On the other hand, the tunnel resistance of the insulating layer (tunnel barrier layer) is maximized when the spinning directions of the two magnetic layers are in opposite parallel as shown in FIG. 19B. This state may be “0” state.
Now, the principle underlying the operation of writing information on the TMR element will be briefly described with reference to FIG. 20.
The TMR element is arranged on the crossing of a write word line WWL and a data selection line (bit line) BL that intersect each other. The writing operation is performed by causing electric currents to flow respectively through the write word line WWL and the data selection line BL and making the spinning directions of the TMR element in parallel or in opposite parallel relative to each other by means of the magnetic field produced by the electric currents flowing through the wires.
When writing data, an electric current is made to flow through the data selection line BL only in a given direction, whereas the electric current that flows through the write word line WWL is made to run in a direction or in the opposite direction depending on the data to be written. The spinning directions of the TMR element are in parallel (in the “1” state) when an electric current is made to flow through the write word line WWL in the first direction, whereas the spinning directions of the TMR element are in opposite parallel (in the “0” state) when an electric current is made to flow through the write word line WWL in the second direction.
The spinning directions of the TMR element are changed in a manner as described below.
Referring to FIG. 21A showing a TMR curve, the resistance of the TMR element changes typically by 17% when a magnetic field Hx is applied to it along a long side (easy-axis) of the TMR element. The rate of the change, or the ratio of the resistances observed before and after the change is referred to as MR ratio.
The MR ratio can vary depending on the nature of the magnetic layers. Currently, TMR elements showing an MR ratio of about 50% are available.
The combined magnetic field of the magnetic field Hx in the direction of the easy-axis and the magnetic field Hy in the direction of the hard-axis is applied to the TMR element. As shown by the solid lines and the dotted lines in FIG. 21A, the magnitude of the magnetic field Hx in the direction of the easy-axis that is necessary for changing the resistance of the TMR element can change depending on the magnitude of the magnetic field Hy in the direction of the hard-axis. It is possible to write data only on the TMR element arranged on the crossing of the selected write word line WWL and the selected data selection line BL out of the memory cells arranged in the form of an array by utilizing this phenomenon.
This will be described further with reference to the asteroidal curve illustrated in FIG. 21B.
FIG. 21B shows a typical asteroidal curve of a TMR element by means of solid lines. If the magnitude of the combined magnetic field of the magnetic field Hx in the direction of the easy-axis and the magnetic field Hy in the direction of the hard-axis is found outside the asteroidal curve (solid lines) (as indicated by black dots), it is possible to reverse the spinning direction of the related magnetic layer.
Conversely, if the magnitude of the combined magnetic field of the magnetic field Hx in the direction of the easy-axis and the magnetic field Hy in the direction of the hard-axis is found inside the asteroidal curve (solid lines) (as indicated by white dots), it is not possible to reverse the spinning direction of the related magnetic layer.
Therefore, it is possible to control the operation of writing data on the TMR element by changing the magnitude of the combined magnetic field of the magnetic field Hx in the direction of the easy-axis and the magnetic field Hy in the direction of the hard-axis as indicated by a position in the Hx-Hy plane.
The data written on a selected TMR element can be read out by flowing an electric current to the TMR element and detecting the resistance of the TMR element.
For example, switching elements are connected to the respective TMR elements in series and only the switching elements connected to the selected read word line are turned on to form an electric current path. As a result, an electric flows only to the selected TMR elements so that it is possible to read the data stored in the TMR elements. Of the accompanying drawing, FIG. 22 is a cross sectional view of a typical magnetic random access memory obtained by applying MOSFETs as switching elements, FIG. 23 is a cross sectional view of another typical magnetic random access memory obtained by applying diodes as switching elements.
However, in the case of memory cells illustrated in FIGS. 22 and 23, each of them occupies a large area because it has a switching element in it.
Cells having no switching element have been proposed by Infineon Technologies AG, in order to reduce the area occupied by each cell. A memory cell array formed on the proposed concept by applying practical design rules may show a cross sectional view as illustrated in FIG. 24.
In the structure illustrated in FIG. 24, a data selection line BL and a read/write word line RWWL are directly connected to each TMR element and a high voltage is applied to a selected TMR element for a write operation. Then, the voltage resisting performance of each TMR will be degraded at an enhanced pace.